This invention relates, generally, to non-aqueous electrochemical cells and, more particularly, to inorganic cells employing an alkaline metal, such as lithium, as the anode, with a cathode collector separated from the anode by a separator membrane.
Among all the known combinations of lithium anodes with different cathodes and electrolytes, those believed to have among the highest energy density and lowest internal impedance use certain inorganic liquids as the active cathode depolarizer. This type of cell chemistry is commonly referred to as "liquid cathode," and it is with respect to this general chemistry that cells of the type disclosed herein are directed.
Early liquid cathode cells use sulfur dioxide as the active cathode depolarizer as described in U.S. Pat. No. 3,567,515 issued to Maricle, et al. on Mar. 2, 1971. Sulfur dioxide is also employed as a solvent in such systems. Liquid cathode cells using oxyhalides are described in U.S. Pat. No. 3,926,669 issued to Auburn on Dec. 16, 1975. As described in Auburn, the anode is generally lithium metal or alloys of lithium and the electrolyte solution is an ionically conductive solute dissolved in a solvent that is also an active cathode depolarizer.
The solute may be a simple or double salt which will produce an ionically conductive solution when dissolved in the solvent. Preferred solutes are complexes of inorganic or organic Lewis acids and inorganic ionizable salts. The requirements for utility are that the salt, whether simple or complex, be compatible with the solvent being employed and that it yield a solution which is ionically conductive. According to the Lewis or electron acceptor theory of acids and bases, many substances which contain no active hydrogen can act as acids or electron acceptors or electron donors. In U.S. Pat. No. 3,542,602 it is suggested that the complex or double salt formed between a Lewis acid and an ionizable salt yields an entity which is more stable than either of the components alone. A typical Lewis acid suitable for incorporation in cells of the type contemplated herein is aluminum chloride which, when combined with a suitable ionizable salt such as lithium chloride, yields lithium aluminum chloride (LiAlCl.sub.4), which is maintained in a suitable solvent such as sulfur dioxide (SO.sub.2).
In addition to an anode, an active cathode depolarizer and ionically conductive electrolyte, cells of this type also require the use of a current or cathode collector. According to Blomgren, as taught in British Pat. No. 1,409,307, any compatible solid, which is substantially electrically conductive and inert in the cell, will be useful as a cathode collector since the function of the collector is to permit external electrical contact to be made with the active cathode material. It was taught to be desirable to have as much surface contact as possible between the liquid cathode and current collector and, as such, most teachings have focused upon the use of a porous material, such as graphite, as the current collector.
It has now been recognized that for a non-aqueous secondary cell, the cathode collector should preferably be inert under certain severe environmental conditions. These include a marked inertness toward the electrolyte solvent solution of, for example, lithium aluminum tetrachloride (LiAlCl.sub.4) in sulfur dioxide (SO.sub.2). This inertness should evidence itself over the voltage range of 2.5 to 4.0 V, while also exhibiting an inertness towards overcharge products.
It is commonplace in non-aqueous secondary electrochemical cells to have cupric chloride present in the electrolyte solution. This is the result of the following reaction: EQU CuCl+AlCl.sub.4.sup.- .fwdarw.CuCl.sub.2 +AlCl.sub.3 +e.sup.-
Appropriate cathode collectors must also be inert towards cupric chloride and its reduced species, as well as display a low ohmic resistance, which is a necessary characteristic of any current collector for use in such an environment.
Unfortunately, very few materials are capable of displaying those necessary characteristics as outlined above. It is thus an object of the present invention to provide a novel rechargeable secondary non-aqueous cell having a current collector which is inert over the voltage range of 2.5 to 4.0 V to solutions of LiAlCl.sub.4 (SO.sub.2), and various overcharge products, as well as towards CuCl.sub.2.